Sizing your battery bank appropriately is perhaps the most critical step when preparing your RV for off-grid living. At a minimum you’ll probably want to run essential electrical equipment for at least one day without recharging. With enough batteries you could even power an electric refrigerator, microwave oven, or an air conditioner. So how many Amp Hours (Ah) of battery capacity do YOU need?
There’s no one-size-fits-all solution here. Arriving at the right battery capacity will involve some trial and error. To start, you’ll need to ask yourself these questions:
- How much energy do you consume on a daily basis?
- How often are you able to recharge your batteries?
- How much space do you have for batteries?
- What is your budget?
How much energy to you consume on a daily basis?
How do you figure this out? Using a detailed spreadsheet to calculate your daily energy usage was the old-school way to determine this, but not many installers do this anymore.
Measuring the actual amount of energy used is another technique which requires special battery monitoring equipment. This is a great strategy if you already have a good battery monitoring system (BMS) installed and are looking to upgrade. This is pretty tough to accomplish though if you don’t have a BMS installed.
Most installers these days focus initially on identifying large energy consumers (equipment like residential refrigerators) that will run continuously throughout the day and night. They are often referred to as continuous loads and will draw more power over time.
Estimating the daily consumption of these large continuous loads is key to developing a realistic power consumption baseline.
Installers and DIYers will then select a standard battery system configuration of 200, 400, or 600 Amp Hours (Ah) and a suitable power inverter to meet the demand of the continuous loads plus smaller on-demand appliances and personal electronics.
Example: An RV with a residential refrigerator that consumes 130Ah per day will need at least 200Ah of usable battery capacity to keep it running. There are likely to be personal electronics that need recharging plus a TV, laptop, lights, etc. A battery bank with 400Ah of usable capacity is a good starting point.
Check out this article for several examples of standard off-grid RV system configurations.
What is usable capacity?
The usable capacity of a battery is the amount of usable energy you can realistically draw from it. It is common to have a usable capacity that is less than the rated capacity. The type of batteries (or battery chemistry) you choose will be the determining factor. Batteries may also degrade over time which will diminish its usable capacity.
We see a big difference in usable capacity when comparing traditional lead-acid batteries to modern Lithium batteries. Popular lithium iron phosphate (LiFePO4) batteries can provide up to 100 percent of their rated capacity. Traditional lead-acid batteries will only provide about 50 percent of their rated capacity and degrade faster as you put more load on them. This is simply a battery chemistry limitation.
Currently most new RVs come equipped with traditional lead-acid batteries. So it’s important to realize this limitation and double the number of lead-acid batteries in your bank in order to achieve the usable capacity you’re looking for.
Example: 200Ah of lithium batteries will provide roughly 200Ah of usable capacity while 200Ah of lead-acid batteries will only provide 100Ah of usable capacity. You’ll need to double the number of lead-acid batteries.
Knowing this limitation of lead-acid batteries, you can see why lithium batteries from popular manufacturers like Battleborn Batteries have become popular upgrades. Lithium batteries can also sustain higher power levels, recharge faster and are one-third the weight of their lead-acid predecessors.
How often are you able to recharge your batteries?
Sizing your battery bank to support a single day of off-grid energy use creates limitations and requires you to replenish that energy each day.
While there are several ways to recharge your batteries, each has it’s own requirements. Recharging from solar panels requires hours of clear skies and sun. Recharging from your engine’s alternator (or use a DC-to-DC charger) requires you to run your engine or hit the road frequently. Even running a generator with a battery charger requires fuel. All of these methods also require time.
Consider these factors when sizing your battery bank. How, where and when do you plan to travel off-grid? You may want to plan for two or three days of reserve capacity. If that’s not an option initially, then I’d recommend designing a setup that can be scaled up if needed.
How much room do you have for batteries?
Space is often a limiting factor for how big you can make your battery bank. Calculating your battery bank requirements may be a no-brainer in this case as you’ll simply fit as many as possible in the space provided.
Consider lithium batteries to make efficient use of your space and provide the most reserve capacity per cubic inch. Don’t forget that you’ll also need room for a power inverter.
It’s best to keep your batteries in the same compartment and use short cable connections for greatest efficiency. If you have to split multiple batteries into separate compartments, keep the distance short and use properly sized cable of the same length to connect the batteries.
What is your budget?
The cost of batteries is often the biggest limitation when upgrading their battery bank. Back in the day, simply adding a second battery to your RV was the thing to do. Nowadays, we really want to power everything off-grid. This requires making a significant investment in batteries that will hopefully last a long time.
Yes. The primary drawback of lithium batteries is that they have a much higher up-front cost compared to lead-acid batteries. On the positive side, they have a longer lifespan which will make them less expensive in the long run.
With that said, I always recommend planning for expansion. Start with a moderate setup that meets your needs. Install a good quality power inverter and battery monitoring system to track your power consumption over time. Then adapt your setup as needed. That’s exactly what I did in our 15 years of RVing.
I started with one lead-acid battery which quickly led to two. Then I switched to 6 volt golf cart batteries, then AGM. A few years ago I finally made the switch to lithium batteries and haven’t looked back. My system now is kind of boring since I don’t have to do anything to it. It just runs and powers our whole coach.
What to read next?
For a comparison of different sized off-grid systems, components and their costs, check out this article. If you’d like to see how I retrofitted the electrical system in our RV, then go to my downloads page and checkout my system diagram.
Of course this is just a simple illustration of how to calculate amp hours. In reality there will be more things drawing power from your batteries than just a single light bulb, but you get the picture.